The water temperature your in tank is one of key the parameters that you want to keep stable. Depending on the quality of your heater, the temperature may fluctuate several tenths of degrees or several degrees before the electronics realize that the heater should be turned on. Some more advanced setups use a high sensitivity temperature probe in conjunction with a switched socket to guarantee the best temperature stability.
When buying a heater, it's important to buy the correct size and wattage. The higher the wattage, the larger the heater tends to be and the more of which the heater needs to be submerged. The higher the wattage, the greater the difference in temperature can be achieved between the desired water temperature and the ambient temperature. If your ambient room temperature is 72 degrees and your desired water temperature is 72 degrees, you won't need as large of a heater! You don't necessarily want an oversized heater either, just in case a fluke problem occurs which causes the heater to turn on full with no shutoff. It's also worth considering having redundant heaters.
Please use our buying guide below! If you need more advice, just ask! Your question may help many other people that are wondering the same thing!
Cobalt Aquatics manufactures the safest and best heater on the market. Their premium heater line, the Neo-Therm series is a slim shatterproof plastic encased heater with simple mechanical buttons to adjust temperature with LED indicators. The temperature range can be set between 66 and 96 fahrenheit. There is built in overload circuitry to shut down the heater if it exceeds safe operating temperatures. The heater is sensitive to within half a degree.